FIG. 1 shows some known headphones 100. Headphones 100 include a front volume 110 formed at one side by the user's ear/head 112 which the driver/speaker of the headphones fires sound into, a driver unit 114 which produces the sound, a rear housing 116, an acoustic resistance R0 which balances audio and passive noise isolation (or ‘passive noise cancellation’) performance, and a driver plate 118 on which a driver is mounted. Headphones 100 may include an acoustic resistance R5 to balance the air pressure between front volume 110 and rear volume 118 to reduce the occlusion effect when the headphones are put on the head of a user. Headphones 100 may comprise cushioning 122 for comfortable fitting of the headphones on the user's head.
Noise isolation performance of known headphones 100 depicted in FIG. 1 is dominated by the acoustic path from acoustic resistance R0 to front volume 110. A noise pressure of P0 in the ambient environment 106 outside headphones 100 results in a noise pressure of P4 in rear volume 118 after passing through acoustic resistance R0. In order to acquire a good noise isolation performance, a dense acoustic resistance R0 is required but the denser the material, the worse the audio performance (especially bass audio performance) is. So, in such known headphones, a good audio (bass) performance contradicts with good noise isolation performance.
Acoustic resistance R0 and acoustic resistance R5 typically comprise acoustic resistive material such as woven mesh, paper mesh or foam material.